Optical lenses are used not only for eyeglasses, but also for various applications, for example, optical systems in various cameras such as cameras, film-incorporated type cameras and video cameras. Examples of the important physical properties of such a lens material include a refractive index (nD) and an Abbe's number (ν). In terms of optical design of an optical unit, in the case where a material with a high refractive index is used, a lens element having a surface with a smaller curvature can be realized. This has advantages in that it can minimize the aberration on this surface, reduce the number of lenses, reduce the eccentricity sensitivity of the lens, and reduce the lens thickness that allows miniaturization and weight reduction of the lens system. In addition, for the purpose of correcting the chromatic aberration, use of a combination of multiple lenses having different Abbe's numbers from each other is known.
As the lens material, optical glasses and optical transparent resins are widely used. Optical transparent resins have advantages such that they allow production of aspheric lenses by injection molding and that they are capable of mass production. Injection molding is a production technique in which plastic is softened by heating, forced into a mold by applying injection pressure so as to fill the mold for molding, left until the resin is cooled, and the molded body is taken out.
Although the fluidity of the resin can be enhanced by increasing the temperature for softening the resin, degradation and coloring of the resin tend to occur and thus the softening temperature is limited. Moreover, although molding die is kept at a constant temperature in many molding machines, if the machine uses pressurized water as a heating medium in a general mold temperature regulator, the mold temperature is about 150° C. at maximum. If surface accuracy of the product is required upon using this machine, the glass-transition temperature of the resin that can be used is limited to about 160° C. at maximum.
Among the optical transparent resins, polycarbonate containing bisphenol A (nD=1.586, ν=30) and polystyrene (nD=1.578, ν=34) are wildly used as high refractive index materials. Their uses vary depending on the difference in the chemical resistance, heat resistance and mechanical characteristics upon heating or the like.
An example of a method for further increasing the refractive index of a plastic material includes the method described in Non-patent Document 1. Theoretically, it is known to introduce an aromatic group or a sulfur molecule into a monomer molecular structure. For example, Patent Document 1 discloses a resin with a refractive index of 1.83. However, although a resin composition having a sulfur atom generally has a higher refractive index, it is known to have a problem of significantly poor light resistance. There are further problems that if the injection molding is performed continuously, the interior of the injection molding machine or the mold would be corroded with the sulfur-containing degradation gas, which renders industrial application difficult and that harmful gas or a sulfur compound is generated when plastic is disposed as waste.
As a material having a high refractive index among the sulfur-free optical resins, Patent Document 2 describes, in Example, a fluorene type polyformal that shows a refractive index of 1.66. Patent Document 3 describes, in Example, polycarbonate having a fluorene structure that shows a refractive index of 1.653 while Patent Document 4 describes, in Example, a copolymer of 2-vinylnaphthalene and styrene that shows a refractive index of 1.6637.
Now, the heat distortion temperature of the fluorene type polyformal that showed the refractive index of 1.66 in Patent Document 2 (Example 2) is high and thus the moldability is moderate, which is not specially good. The polyformal with good moldability (Example 1) did not give a sufficiently high refractive index. While Patent Document 3 discloses a polycarbonate resin, the polycarbonate showing the refractive index of 1.653 has a glass-transition point of 175° C. (Example 1) and one with a glass-transition temperature of 161° C. has a refractive index of 1.646 (Example 3). Patent Document 4 discloses a vinyl resin, where the resin with a refractive index of 1.6637 has a glass-transition temperature of 134.6° C. (Example 4). This copolymer is a copolymer of 2-vinylnaphthalene and styrene, which is a compound whose functional group on the chemical structural formula greatly differs from that of polyformal.
Next, prior art documents that describe compounds similar to the present invention in terms of chemical structural formulae will be illustrated. Patent Document 5 discloses a polycarbonate resin having repeat units derived from 9,9-bis(4-hydroxy-3-phenylphenyl)fluorene (hereinafter, also abbreviated as OPPFL). However, although these polycarbonate resins are described to have high glass-transition temperatures and thus high heat resistance, there are no description about their refractive indexes and birefringences.
For example, OPPFL is used to polymerize a polycarbonate in Patent Document 6. The resultant has a high refractive index of nd=1.656 as a homopolymer but also has high Tg, which is unsuitable as a molding material. In order to decrease Tg, copolymerization with bisphenol A or bis(4-hydroxy-3-methylphenyl)sulfide (hereinafter, abbreviated as HMPS) has been conducted. In a case of a copolymer whose moldability is kept at Tg=120° C. to 160° C., the refractive index value is relatively low.
An example of polyformal using a fluorene skeleton monomer is described in Patent Document 7. However, there is no example showing the use of 9,9-bis(4-hydroxy-3-phenylphenyl)fluorene. With respect to the physical properties, only heat resistance and photoelastic coefficient are mentioned, and there is no description about refractive index that is required as an optical lens.
Other examples using polyformal are described in Patent Documents 8 to 13 but there is no description about the refractive index.
Other examples that use polyformal and mention about the refractive index are described in Patent Document 2 as mentioned above and Patent Document 14. However, in these examples, when the refractive index value nd is relatively high, Tg is high as well, and thus requires a molding machine separately from the molding machine equipped with the above-mentioned general mold temperature regulator that uses pressurized water as a heating medium.